Natural lipid A has mitogenic activity, i.e., an activity to stimulate lymphocytes to cause blast transformation, which accelerates increase of lymphatic cells thereby to enhance immunity, an activity to derive a tumor necrosis factor, and the like, and is, therefore, promising as a treating and prophylactic agent for many diseases caused by a reduction of immune function, such as various infectious diseases, or antitumor agents.
Known derivatives of natural lipid A include those described in Japanese Patent Application (OPI) Nos. 48497/84, 53295/86, and 227586/86 (the term "OPI" as used herein means "unexamined published Japanese patent application"). Among them, 2-deoxy-6-O-(2-deoxy-2-[(R)-3-dodecanoyloxytetradecanoylamino]-4-O-phospho no-3-O-[(R)-4-tetradecanoyloxytetradecanoyl]-.beta.-D-glucopyranosyl)-3-O-[ (R)-3-hydroxytetradecanoyl]-2-[(R)-3-hydroxytetradecanoylamino]-1-O-phospho no-.alpha.-D-glucopyranose disclosed in Japanese Patent Application (OPI) No. 53295/86 (hereinafter referred to as Compound A) is known to have physiological activities equal to or even higher than natural lipid A as reported in Eur. J. Biochem., Vol. 148, 1-5 (1985). Compound A, however, is of low practical use due to high toxicity similar to natural lipid A. The above-described known compounds other than Compound A are also unsatisfactory for practical use in terms of toxicity or antitumor activity. It has been keenly demanded, therefore to develop compounds exhibiting useful physiological properties with reduced toxicity.